1. Field of the Invention:
The present invention is directed generally to sensors and more particularly to sensors used to detect vibrations in rotary machines.
2. Description of the Prior Art:
Historically, the vibrational modes of steam turbine blades have been measured by placing strain gages on the rotating blades and telemetering the information to a stationary receiver. That method is not very desirable because the gages have short lives. It is also necessary to mount a strain gage on each blade which is to be monitored. For those reasons other types of sensors have been investigated.
In U.S. Pat. No. 4,593,566 a capacitive type of sensor is disclosed. As each blade tip passes the sensor, there is a capacitance change in the capacitor defined by the sensor and the blade. This results in small currents flowing into and out of the thus defined capacitor. Those current signals are passed through two threshold detectors. The first, a variable positive level comparator, gates a second close-to-zero level comparator. This allows precise timing through the zero-crossing when the signal-to-noise ratio is high but inhibits the zero crossing comparator during periods when a blade tip is not adjacent a sensor and the signal-to-noise ratio is zero.
Variable capacitance sensors, although inherently fast and rugged, do not give usable results in two-phase flow. Changes in capacitance due to flow induced changes in dielectric properties can cause spurious results.
Optical sensors have been used for blade vibration monitor applications. However, optical sensors are not suitable for long term installation in an operating steam turbine environment because of poor performance in steam containing droplets, refraction effects of surface water, erosion, and contamination of the optical surfaces by oxides and other particles.
Self-generating types of sensors, such as magnetic sensors, have also been investigated. The precision of timing necessary for measuring significant vibration is on the order of forty nanoseconds, which implies a clock frequency of twenty-five megahertz. Nominal response frequencies of magnetic sensors and amplifier circuits are typically less than twenty-five kilohertz. That discrepancy of three orders of magnitude in performance has discouraged the use of such sensors. Accordingly, the need exists for an apparatus capable of precisely detecting the passing of the blades of a rotating machine past a stationary sensor.